The invention relates to particle or cell sizing and counting apparatus and to methods of operation thereof. In particular, the invention relates to apparatus which uses a technique of measuring the impedance at an orifice to determine the volume of a particle passing through the orifice.
It is known from EP 0162607 to determine the size of a particle from the variation in impedance between a pair of electrodes in an electrolyte due to particle flow through an orifice in a flow restrictor disposed between the electrodes. An inherent problem of this type of system however, is that partial or complete blockage of the orifice can occur during measurements which requires that the flow restrictor comprising the orifice must be removed in order to be cleaned to allow further measurements. Additionally, if only partial blockage occurs the observed distribution of particle sizes in a sample will be affected due to prevention of flow by the blockage of larger particles through the orifice. This problem is particularly prevalent if one wishes to use a small orifice diameter of say five times the average particle size to enable good accuracy of particle sizing results.
The invention seeks to avoid or at least mitigate problems of the prior art including providing apparatus which detects blockage and or deblocks the orifice especially in the event of partial or complete blockage.
According to first aspect of the invention there is provided apparatus for determining the size of a particle or cell within a fluid, comprising a sample chamber for the fluid, a flow restrictor comprising an orifice, a pair of electrodes disposed on opposite sides of the orifice and means for measuring a signal representative of the impedance variation between the electrodes thereby to determine the size of a particle within the fluid passing through the orifice, and further comprising means for detecting blockage of the orifice, whether partial or complete. Beneficially the detection of a partial or complete blockage alerts the user to data corruption.
Preferably, the detecting means comprises means for monitoring the signal, which means detects occurrence of a predetermined variation of the signal indicative of blockage of the orifice. For example, particle passage through the orifice causes a signal pulse which is measured by the measuring means and wherein the monitoring means determines a width of the signal pulse and compares this width with a predetermined value thereby to detect partial or complete blockage of the orifice. The predetermined pulse width value can be determined from an average of previously measured pulse widths.
In another form, the monitoring means monitors the mean base line value of the signal to determine if a significant drift in mean base line value occurs which is indicative of partial or complete blockage of the orifice.
Alternatively or as well, the monitoring means compares the height of an individual signal pulse with a known value corresponding to a particle size in the order of or greater than a predetermined size such as the diameter of the orifice. Also, the monitoring means can comprise a saturation pulse or square wave detector. The square wave detector can compare the time for a pulse signal to pass through a first and second predetermined value and then return back through the predetermined first and second values. The square wave detector can be arranged to detect a characteristic recovery curve indicative of saturation of the measuring means.
In another form, the monitoring means measures the rate of occurrence of signal pulses and compares this rate with a predetermined rate. Also, the monitoring means can compare the number of detected signal pulses in a given time interval with a predetermined value, such as an average of previous measurements.
Beneficially, the monitoring means can analyse the background noise of the signal for predetermined variation, such as amplitude variation within a frequency range.
Further, the detecting means can comprise an orifice current detector for determining electrical current flow between the electrodes thereby enabling the monitoring means to compare orifice current with predetermined values. The detecting means can compare the orifice current value before or after measuring a signal or the mean of the two with an initial value measured before the signal measurement. The detecting means can determine if the difference is greater than a 10% increase or a 5% decrease, or if the difference is greater than 20% say.
Preferably, the measuring means comprises a 14 bit detector, or an analogue to digital converter having 14 bit resolution. Also, the apparatus preferably comprises means for applying a calibration signal to one of the electrodes and the monitoring means the signal across both electrodes, the monitoring means being adapted to compare the detected calibration signal with a pre-determined signal. The calibration signal can comprise a series of pulses. The calibration signal can be repeated a predetermined number of times to attempt to obtain an acceptable result with the predetermined signal, before providing an alarm to the user. Also, means for deblocking the orifice by removing a particle at least momentarily held within the orifice can be provided.
According to another aspect of the invention there is provided apparatus for determining the size of a particle or cell within a fluid, comprising a sample chamber for the fluid, a flow restrictor comprising an orifice, a pair of electrodes disposed on opposite sides of the orifice and means for measuring a signal representative of the impedance variation between the electrodes thereby to determine the size of a particle within the fluid passing through the orifice, and further comprising means for deblocking the orifice by causing movement of a particle held at least momentarily within the orifice. Beneficially the deblocking means can effect in situ removal of the blockage, therefore, the apparatus allows continuous sampling without requiring the user to dismantle the sample chamber or remove the flow restrictor for cleaning when a blockage occurs.
Preferably, the deblocking means operably creates ultrasonic vibrations in the fluid in the region of the orifice. A series of electrical pulses can be applied to one of the pair of electrodes. The pulse frequency is preferably greater than 15 kHz, and preferably up to about 20 kHz. Beneficially, the flow restrictor can comprise material that exhibit the piezo-electric effect, thus to enhance the deblocking effect in the presence of electrical pulses.
Also, the deblocking means can comprise fluid propulsion means for effecting fluid flow at the flow restrictor. Preferably, the fluid propulsion means directs fluid substantially in the reverse direction to fluid flow during signal measurements.
Beneficially, means within the chamber can be used for directing fluid from the fluid propulsion means towards the flow restrictor. For example, a tapered region of the chamber can be used.
Another aspect of the invention provides a sample chamber housing for particle sizing apparatus comprising a pair of recesses each adapted to receive an electrode, an electrode in each recess, and a two-part sealant and adhesive for the electrodes. The adhesive can comprise a structural acrylic adhesive. The sealant can comprise a silicone based sealant such as RTV silicon rubber. The recess is preferably defined in a body of a polymer such as acrylic.
A further aspect of the invention provides a flow restrictor for particle sizing apparatus, comprising an orifice allowing flow of particles therethrough which orifice is tapered. The taper is preferably in the order of 5 to 30% of the mean orifice diameter.
A further aspect of the invention provides a flow restrictor wherein the material defining the orifice is flexible.
A yet further aspect of the invention provides use of a 14 bit analogue to digital converter in particle sizing apparatus.